Soot sensors, also known as particulate matter (PM) sensors, are often used in vehicles having diesel engines. A particulate matter sensor may be located upstream from a particulate filter, where the sensor is exposed to exhaust flow from the engine having soot particles entrained in the exhaust gas. Alternatively, a particulate matter sensor may be located in a gas stream downstream from a particulate filter, where the sensor is used to monitor the proper operation of the particulate filter.
A known method of sensing soot uses a particulate matter sensor having two electrodes that are spaced from one another. In the absence of soot, there is very low electrical conductivity between the electrodes. As soot accumulates on the surface of the sensor, soot particles act to bridge the gap between the electrodes. Because the soot particles are electrically conductive, the conductivity between the electrodes increases, and this change in conductivity can be related to the amount of soot in the gas stream. Sensors that operate according to this principle are disclosed in U.S. patent application Ser. No. 11/749,262 published as US Patent Application Publication 2008/0283398, U.S. patent application Ser. No. 11/750,883 published as US Patent Application Publication 2008/0282769, and U.S. patent application Ser. No. 11/998,238 published as US Patent Application Publication 2009/0139081, the contents of all of which are hereby incorporated by reference in their entirety.
Government regulations require that the particulate matter sensor have self diagnostics (i.e. On Board Diagnostics or OBD) capability to verify that it is functioning properly. However, with a normally open circuit device, and soot normally not present, this can be difficult. The sensor must be able to verify that the circuit is functioning properly and that if a conductive material lands on the electrode, the sensor can detect it. In a conventional sensor as described, a “clean” sensor, that is a sensor with no accumulated soot, will appear electrically as an open circuit. The same open circuit indication may result from a damaged sensor or a disconnected wiring harness.
Accordingly, the inventors herein have recognized a need for an improved sensing system having a particulate matter sensor that reduces and/or eliminates the foregoing deficiencies.